Hydrogenation of CO2 in Water Using a Bis(diphosphine) Ni–H Complex

Changing the Mechanism for CO2 Hydrogenation Using Solvent-Dependent Thermodynamics

A critical scientific challenge for utilization of CO₂ is the development of catalyst systems that function in water and use inexpensive and environmentally friendly reagents. We have used thermodynamic insights to predict and demonstrate that the HCo^I (dmpe)₂ catalyst system, previously described for use in organic solvents, can hydrogenate CO₂ to formate in water with bicarbonate as the only added reagent. Replacing tetrahydrofuran as the solvent with water changes the mechanism for catalysis by altering the thermodynamics for hydride transfer to CO₂ from a key dihydride intermediate. The need for a strong organic base was eliminated by performing catalysis in water owing to the change in mechanism. These studies demonstrate that the solvent plays a pivotal role in determining the reaction thermodynamics and thereby catalytic mechanism and activity.

A Bimetallic Nickel-Gallium Complex Catalyzes CO2 Hydrogenation via the Intermediacy of an Anionic d10 Nickel Hydride

Large-scale CO₂ hydrogenation could offer a renewable stream of industrially important C₁ chemicals while reducing CO₂ emissions. Critical to this opportunity is the requirement for inexpensive catalysts based on earth-abundant metals instead of precious metals. We report a nickel-gallium complex featuring a Ni(0)→Ga(III) bond that shows remarkable catalytic activity for hydrogenating CO₂ to formate at ambient temperature (3150 turnovers, turnover frequency = 9700 h^-1), compared with prior homogeneous Ni-centered catalysts. The Lewis acidic Ga(III) ion plays a pivotal role in stabilizing catalytic intermediates, including a rare anionic d¹⁰ Ni hydride. Structural and in situ characterization of this reactive intermediate support a terminal Ni-H moiety, for which the thermodynamic hydride donor strength rivals those of precious metal hydrides. Collectively, our experimental and computational results demonstrate that modulating a transition metal center via a direct interaction with a Lewis acidic support can be a powerful strategy for promoting new reactivity paradigms in base-metal catalysis.

Renewable Formate from C-H Bond Formation with CO2: Using Iron Carbonyl Clusters as Electrocatalysts

As a society, we are heavily dependent on nonrenewable petroleum-derived fuels and chemical feedstocks. Rapid depletion of these resources and the increasingly evident negative effects of excess atmospheric CO₂ drive our efforts to discover ways of converting excess CO₂ into energy dense chemical fuels through selective C-H bond formation and using renewable energy sources to supply electrons. In this way, a carbon-neutral fuel economy might be realized. To develop a molecular or heterogeneous catalyst for C-H bond formation with CO₂ requires a fundamental understanding of how to generate metal hydrides that selectively donate H^- to CO₂, rather than recombining with H⁺ to liberate H₂. Our work with a unique series of water-soluble and -stable, low-valent iron electrocatalysts offers mechanistic and thermochemical insights into formate production from CO₂. Of particular interest are the nitride- and carbide-containing clusters: [Fe₄N(CO)₁₂]^- and its derivatives and [Fe₄C(CO)₁₂]^2-. In both aqueous and mixed solvent conditions, [Fe₄N(CO)₁₂]^- forms a reduced hydride intermediate, [H-Fe₄N(CO)₁₂]^-, through stepwise electron and proton transfers. This hydride selectively reacts with CO₂ and generates formate with >95% efficiency. The mechanism for this transformation is supported by crystallographic, cyclic voltammetry, and spectroelectrochemical (SEC) evidence. Furthermore, installation of a proton shuttle onto [Fe₄N(CO)₁₂]^- facilitates proton transfer to the active site, successfully intercepting the hydride intermediate before it reacts with CO₂; only H₂ is observed in this case. In contrast, isoelectronic [Fe₄C(CO)₁₂]^2- features a concerted proton-electron transfer mechanism to form [H-Fe₄C(CO)₁₂]^2-, which is selective for H₂ production even in the presence of CO₂, in both aqueous and mixed solvent systems. Higher nuclearity clusters were also studied, and all are proton reduction electrocatalysts, but none promote C-H bond formation. Thermochemical insights into the disparate reactivities of these clusters were achieved through hydricity measurements using SEC. We found that only [H-Fe₄N(CO)₁₂]^- and its derivative [H-Fe₄N(CO)₁₁(PPh₃)]^- have hydricities modest enough to avoid H₂ production but strong enough to make formate. [H-Fe₄C(CO)₁₂]^2- is a stronger hydride donor, theoretically capable of making formate, but due to an overwhelming thermodynamic driving force and the increased electrostatic attraction between the more negative cluster and H⁺, only H₂ is observed experimentally. This illustrates the fundamental importance of controlling thermochemistry when designing new catalysts selective for C-H bond formation and establishes a hydricity range of 15.5-24.1 or 44-49 kcal mol^-1 where C-H bond formation may be favored in water or MeCN, respectively.

CO2 reduction or HCO2− oxidation? Solvent-dependent thermochemistry of a nickel hydride complex

The hydricity (ΔG_H−) of a newly synthesized nickel hydride was experimentally determined in acetonitrile (50.6 kcal mol⁻¹), dimethyl sulfoxide (47.1 kcal mol⁻¹), and water (22.8 kcal mol⁻¹).